Question

Select the three most acidic hydrogen atoms in lactic acid, CH3CH(OH)CO2H, and rank them in order of decreasing acidity. Explain your reasoning.

Answer 1

The three most acidic hydrogen atoms in lactic acid, CH₃CH(OH) CO2H in order of decreasing acidity: COOH, -OH, -CH, -CH₃

Further explanation

In determining the acid strength of a Hydrogen, we must look at the conjugate bases after we take the Hydrogen atom.

The more stable anion that is formed after we take the hydrogen atom, the easier it is to lose hydrogen and the more acidic the hydrogen is

The effect of the stability of this anion can be influenced by the value of electronegativity, induction or resonance

Another method that we can use to find the Hydrogen acidity value is the pKa value in the functional group to which Hydrogen is bound

From the existing pKa values, they include:

sp³ carbon = pka 50

sp² carbon = pka 45

sp carbon = pka 25

amine = pka 35

alcohol = pka 16

phenol = pka 10

carboxylic acid = pka 5

hydrogen halide = pka 3 to - 10

From lactic acid compounds there are 4 functional groups of H-bound protons, namely: COOH, -OH, -CH, -CH₃

If we look at the value --- pKa (-logKa) is inversely proportional to the acidic strength of the compound --- then the order of acidity of Hydrogen from the strongest to the weakest is:

• 1. COOH pKa 5

• 2. OH ​​/ alcohol pKa 16

• 3. CH pKa 25

• 4. CH₃ pka 50

If we see COOH has a stronger acidity than OH because the conjugate base COO⁻ is more stable than O⁻ because negative ions are more evenly distributed because their ionic forces are attracted by 2 oxygen atoms

While OH and CH itself in terms of electronegativity, O atoms are more electronegative compared to C atoms, so the conjugate bases are more stable (negative ions), so alcohol is more acidic than CH

And CH₃ itself is the most unstable conjugate base among others so that the acidity is the smallest

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Keywords: lactic acid, resonance, pKa, induction. electronegativity

Answer 2

It is easier to determine this if we draw the structural formula of lactic acid as shown in the attached picture. There are three functional groups in lactic acid: carboxyl group, hydroxyl group, and the parent alkane chain. Any of the hydrogens in the alkane chain is the least acidic. Then, it is followed by the H in the hydroxyl group. The most acidic is the H in the carboxyl group.